The past decade novel coding techniques have appeared for the coding of still images, inter alia techniques based on the principle of segmentation of an image into a set of regions while preserving, however, the contours corresponding to the objects present within the image. The article "Recent results in high-compression image coding", by M. Kunt, M. Benard and R. Leonardi, published in IEEE Transactions on Circuits and Systems, Vol. CAS-34, No. 11, November 1987, pp. 1306-1336, describes the principles for carrying out this technique in its section IV "Adaptive split-and-merge".
Generally speaking, the technique for coding an image by segmentation consists in determining, according to given criteria, regions within the image that may be considered to be comparatively homogeneous (that is to say with small or zero variation of the brightness) and searching for a symbolic representation of the regions thus determined, while preserving the information relating to the contours of said regions. Generally speaking, this symbolic representation is obtained by searching for at least an approximation of characteristics which are specific of each of these regions, and coding with a rather high compression ratio can then be achieved by applying the coding procedure to the symbols thus defined.
More specifically, the actual segmentation obtained by searching for a mathematical or statistical model which correctly translates the spatial variations of the brightness (or grey level) of the image which is realised, for example by means of low-order polynomial functions. In the present case where the regions are comparatively homogeneous, the coding of the information representative of the contents of these regions does not pose major problems. The shape and the position of each region are defined by their contour which must be coded in a reliable manner. Exact coding of each complete contour is, therefore, the simplest means for reliably transmitting the chain of successive points constituting a contour, for example by supplying the address of a first point of the contour by way of its real coordinates, by deriving therefrom the subsequent points by a sequence of information taken from three possible symbols: turn left, turn right, go straight ahead; this suffices to construct the contour chain.
Unfortunately, even though at present methods are available which seem satisfactory for coding, with data compression, of the texture of image regions or, more generally speaking, their contents, this does not hold for the contours which, since they are more numerous as the regions are more numerous, remain very costly in respect of coding because they represent a major part of the signals to be coded.